The field of the disclosure relates generally to the effects of sloshing within a tank of fluid on a ship, and more particularly, to methods and systems for providing sloshing alerts and advisories.
Sloshing describes the phenomenon of a liquid inside a tank where the liquid is excited by the motion of the vehicle carrying the tank. For example, on large liquid natural gas (LNG) carriers, sloshing of the LNG due to ship motion in a seaway can lead to extremely high loads on the cargo tank walls, resulting in extensive damage to the tank structure or insulation material. The cargo tank is particularly susceptible to damage when it is partially filled, and further susceptible to damage when the natural period of the liquid in the tank is in resonance with (near or equal to) roll or pitch motion periods of the ship.
Sloshing is stochastic, meaning that impacts cannot be predicted with certainty and the magnitudes of such impacts can vary widely. There are several factors influencing the severity of a sloshing response, including tank design configuration and local details such as chamfered topsides fabricated within the individual tanks, the fill level of the tanks, as well as excitation motion characteristics, including period, magnitude and duration. Factors influencing vessel motion include the loading condition of the vessel (i.e. the metacentric height (GM) and draft of the ship), inertia properties of the vessel about its axis of rotation, free surface effects (i.e. partially filled tanks effectively reduce GM), damping from ship appendages, as well as ship speed and heading relative to incoming waves.
As mentioned above, some LNG tanks are configured with a chamfered topside and hopper bottoms to reduce the chance of resonance while the tank is more than 95% full during a loaded passage, or less than 5% fill during the ballast leg. Loading and unloading operations usually take place at terminals or docks in protected water with good weather. As the LNG sector expands, larger tank sizes in larger vessels are built with new trades, which may entail the vessel operating with partially filled tanks and conducting cargo operations in more exposed waters. The increases in size and the need for partial fill loads also result in changes to tank natural periods. Certain of these periods are in range with roll and pitch periods of such ships. Coupled with increased severity of sea states on some trade routes, the risk of cargo and ship damage has significantly increased, especially when liquid sloshing occurs due to resonance with ship motions.
As an example, damage has occurred to several membrane tank LNG carriers due to sloshing of the LNG cargo. Lower filling levels in these LNG carriers can actually produce higher sloshing loads. Further, the study of sloshing is complex as many aspects are not easily addressed by calculations and testing. Computational fluid dynamic determinations are not fully reliable as they do not consider entrapped bubbles. As a result of recent sloshing damage incidents, regulations directed to lower filling heights for membrane ships have been reduced twice over a relatively short period. However, these reductions in cargo greatly restrict the flexibility LNG transporters have in dispensing partial loads at multiple sites. As a result, LNG transporters are currently restricted to travelling only with practically full or practically empty loads.